1. Field of the Invention
The present invention relates to a substrate for a light-emitting device and an optical device using the same, and the optical device of the present invention is used for a display device such as display. In addition, the present invention is applied to an encapsulating member for an organic electroluminescent (EL) device.
2. Description of the Related Art
An organic electroluminescent (EL) device is an emissive device using a principle that a fluorescent material emits light by means of recombination energy of holes injected from an anode and electrons injected from a cathode when an electric field is applied thereto. Researches have been conducted with regard to the organic EL device comprised of organic material after a stacked type low voltage driving organic EL device was disclosed in the art. Tang et al uses tris(8-quinolinol)aluminum for an emission layer, and uses triphenyldiamine for a hole transporting layer. The stacked type structure has an advantage in that efficiency of injecting the hole into the emission layer may be increased, efficiency of forming an exciton resulted from recombination by blocking the electron injected from the cathode may be increased, and the exciton formed in the emission layer may be blocked. For the structure of the organic EL device as mentioned above, two-layer type of a hole transporting (injecting) layer and a hole transportable emission layer, or three-layer type of a hole transporting (injecting) layer, an emission layer and an electron transporting (injecting) layer are well known in the art. In order to increase the recombination efficiency of the injected electron and hole in the stacked type device, researches for the device structure or a method for forming the same are conducted. However, the organic EL device has a limitation for the probability of singlet generation due to dependency of spin statistics when carriers are recombined, which causes to have an upper limit of a luminescence probability. The value of this upper limit is known to be about 25%.
In the surface emitting device having spherical-wave-shaped wavefront such as the organic EL device with at least emission layer interposed between an anode and a cathode, a refractive index of the luminous body is higher than a substrate or air, so that the light of an exiting angle more than a critical angle is totally reflected at a substrate/air interface or the like, which causes the light not to be extracted outside the substrate. It is expected that only 20% of the total amount of light emitted is available when the refractive index of the luminous body is 1.6. As such, energy converting efficiency becomes low, which is limited to be about 5% in total when singlet formation probability is added to. In the organic EL device that has a luminescence probability highly limited, low light extraction efficiency as mentioned above causes degradation of the energy converting efficiency, which may be severely and adversely affected.
As for the technique of improving the light extraction efficiency, several proposals have been suggested in the prior art.
Japanese patent publication No. S63-314795 discloses a method for forming a lens on a substrate. A substrate having a focusing property such as selfoc or convex lens is used for the disclosure.
Japanese patent publication No. H01-220394 discloses a method for forming a reflective surface as a technique for improving the light extraction efficiency. This publication is characterized in that a mirror for reflecting light is arranged at one side of an emission layer, and the mirror is shaped to be a mortar to prevent light loss around the emission layer.
Japanese patent publication No. 2001-202827 discloses a method for arranging a low refractive layer between a substrate and an electrode layer. According to this disclosure, a transparent conductive layer (namely, electrode layer) is formed in contact with at least one surface of the low refractive layer, so that light transmitting the low refractive layer may be highly extracted out to the air, and the extraction efficiency for extracting the light out of the structure as mentioned above becomes higher, and the refractive index of the low refractive layer is 1.003 to 1.300, so that the light transmitting the low refractive layer becomes highly extracted out to the air, and the light extraction efficiency for taking the light out of the structure becomes higher, and silica aerogel is used for the low refractive layer, which leads to implement an ultra low refractive index close to 1.
Japanese patent publication No. 2003-31374 discloses a light-emitting device having an anti-reflective layer that comprises a high refractive layer, a low refractive layer, and a high refractive layer in this order between an emission layer and a substrate. In accordance with this disclosure, FIG. 1 shows an optical multilayer body 2 comprised of a high refractive layer H1, a low refractive layer L1, and an anode layer 2A as a high refractive layer in this order between a substrate 1 made of a glass and an organic EL layer 3. And paragraph number [0025] thereof describes that the optical multilayer body 2 has an anti-reflective property and the light extraction efficiency from the substrate 1 is improved.
When the anti-reflective layer is formed of a single layer, reflective light is canceled off by light interference when nd=λ/4 (wherein d is thickness of physical layer and λ is wavelength to used) is met. The invention of the above-mentioned disclosure relates to one kind of such anti-reflective layer, and has a multi layered structure. Thickness of the high refractive layer H1, low refractive layer L1 and anode electrode layer 2A are 14.2 nm, 41.5 nm, and 139.8 nm, respectively, and the thickness of each layer is set to be not more than half wavelength of the emission wavelength of 400 to 700 nm in the structure described in its embodiment (see paragraph No. 0015). The multi layer described in this publication is one kind of so called anti-reflective layer.
However, these prior arts still need to be improved.
The method for forming lens or reflective surface on a substrate described in Japanese patent publication No. 1989-314795 or 2001-220394 is effective in a device having is a large emission area, but not suitable for forming lens with a focusing property or side reflective surface in a device having a fine pixel area such as dot matrix display. In particular, layer thickness of the organic EL device is several μm or less, forming a reflective mirror to be a tapered shape at a side of the device is not possible with a current technique for fine processing, which obviously and significantly causes high cost.
The method for interposing a low refractive layer between a substrate and an electrode layer as described in Japanese patent publication No. 2001-202827 is effective in terms of improving light extraction efficiency by collecting light within a critical angle, however, the light becomes reflected from an interface between an anode electrode and the low refractive layer, so that it is not sufficient for improving the light extraction efficiency. In addition, when a porous silica aerogel layer is used so as to obtain an ultra low refractive layer, mechanical intensity of the layer becomes very weak. Furthermore, an electrical short circuit occurs due to surface unevenness of the porous layer, which causes non-emission region (namely, dark spot) to be occurred. As such, the technique for light extraction still needs to be improved for the organic EL device.
The method for arranging the anti-reflective layer as described in the Japanese patent publication No. 2003-31374 has been limited to improve the light extraction efficiency in terms of its function (which will be described in the embodiment paragraph later).
In addition, as is well known in the art, the anti-reflective layer is highly dependent on wavelength, so that the light extraction efficiency is greatly varied due to the emission wavelength. Because of this factor, when the technique described in the same publication is applied to a white emitting device, the amount of light emitted outside the substrate is dependent on wavelength, which causes the amount to be greatly different from one is another based on the wavelength, so that white balance is degraded.
Furthermore, the anti-reflective layer is one that uses light interference to cancel off the light reflection, it needs to be formed to have its layer thickness or refractive index met with predetermined conditions. Thus, when the layer thickness is finely varied according to fabrication factors, the reflectivity of the anti-reflective layer is varied, which readily causes variation of the device performance.
Development of the light extraction technique is of major interest for applying it to a low voltage drive of light-emitting device such as organic EL device, and is inevitable for implementing low power consumption of the device.